9*2 PALJEONTOLOGY OP NEW-YORK. 



from Port Colborne, Canada West, which is apparently identical with it. The finest 

 and best characterized specimens which I have seen are from the Falls of the 

 Ohio ; and it occurs in almost equal perfection at Columbus and Sandusky, Ohio. 



Stropliodoiita pcrplana. 



PLATES XI & XIJ. 



Strophomena perplana : Conbad, Jour. Acad. Nat. Sci. Philadelphia, Vol.vHi, p. 257. 1842. 



Stropkomena crenUtria : Uall, Gcol. Rep. 4th District, p. 171, f. 4. 



For description and references, sec the same species under the Hamilton group. 



A comparison of large numbers of specimens of this species, from the 

 base of the Lower Helderberg to the Hamilton and Chemung groups 

 inclusive, embracing those described under several different names, has 

 satisfied me that they must all be referred to a single species. There 

 is, however, a similar species in the TuUy limestone, which in the cha- 

 racter of its surface striae, the pustulose or punctate interior surface of 

 the valves, and the form of the muscular impressions, is very distinct 

 from this oqe. ^ 



This species begins its existence, so far as at present known, at the commence- 

 ment of the Schoharie grit, where it is not infrequent. It occurs as casts of the 

 interior ; those of the dorsal valve being the more common. It appears in the 

 Corniferous limestone, where it attains large dimensions. It was a ventral valve of 

 this species imbedded in limestone, having the surface rather better preserved 

 than usual, to which I originally gave the name of S. crenistria. 



The figure 22 of Plate xi is a cast of the ventral valve from the Schoharie grit. 



Figures 13 & 14 of Plate xil are a small and large specimen of the shell, with nearly 

 even striae, as it occurs in the Corniferous limestone. 



Figure 15 is a cast of the same species, where the muscular impression is very large. 



Geological formation and locality. In the Schoharie grit, at Clarksville and 

 Knox, Albany county and at Schoharie, in the limestone at Williamsville and 

 Clarence-hollow, Erie county ; at Louisville, Kentucky ; and in Indiana. 



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